A Syrian government envoy has denounced a speech by a rebel leader as "provocative" and "insolent" at peace talks in Kazakhstan.
Bashar Ja'afari, Syria's UN ambassador, said rebel leader Mohammad Alloush's speech in Astana did not rise to the level of the gathering of diplomats attending the conference.
Mr Ja'afari repeatedly referred to the rebel delegation as representatives of "terrorist armed groups". He also said that the agenda for the talks, which are sponsored by Russia, Turkey and Iran, is "not ready yet".
The harsh and uncompromising tone of Mr Ja'afari's remarks do not augur well for the summit, which had barely started following an opening ceremony and speeches by various representatives.
In his speech, Mr Alloush described Syrian president Bashar Assad's government as "terrorist" and called for armed groups fighting alongside the Syrian army, including Lebanon's Hezbollah group, to be placed on a global list of terror organisations.
Mr Alloush said: "The presence of foreign militias invited by the regime, most notably the Lebanese Hezbollah and the Iraqi Hezbollah ... contributes to the continuation of bloodshed and obstructs any opportunity for a ceasefire."
He added that Mr Assad's forces and their allies are no different to Islamic State, and should be designated "terrorist groups".
The UN envoy to Syria, Staffan de Mistura, is participating in the talks in the Kazakh capital, which, if successful, are expected to be followed by another summit in Geneva next month.
The new US government is not directly involved, because of the "immediate demands of the transition", but Washington is represented by the US ambassador to Kazakhstan, George Krol, who attended Monday's opening session at the luxury Rixos President Hotel in Astana.
Earlier, Osama Abo Zayd, a rebel media representative, said the scope of the negotiations is limited to strengthening the current ceasefire arrangement.
He said: "There's no significance to negotiations if the people on whose behalf we are negotiating are being killed", adding that there has been absolutely no discussion about elections or Mr Assad's future.
The Syrian civil war is estimated to have killed about 400,000 people since March 2011. The conflict, which started as an uprising against Mr Assad's rule against the backdrop of Arab Spring movements, quickly descended into all-out civil war.
The war has displaced half of Syria's population and sent millions of refugees to neighbouring countries and Europe.
After a short opening ceremony during which Kazakh foreign minister Kairat Abdrakhmanov spoke, the meeting went into closed session. It was not immediately clear if there would be any direct talks between the rebels and Damascus representatives behind closed doors.
At the top of the agenda is an effort to consolidate last month's ceasefire brokered by Turkey and Russia. The truce, which excludes extremist groups such as Islamic State and the al-Qaida affiliate in Syria, has reduced overall violence although fighting and violations continue on multiple fronts.
In Tehran, Iranian foreign ministry spokesman Bahram Ghasemi said preserving the tenuous ceasefire will be "the most important issue" on the Astana agenda and that Iran is hopeful the talks can also pave the way for the delivery of humanitarian aid.
Mr Ghasemi suggested discussions over a larger political settlement would have to wait, saying: "Let's wait and see how the process can be continued based on conclusions that will be announced on Tuesday."
The talks, organised by Russia and Turkey, are the latest attempt to halt the near six-year conflict. Russia and Iran are the main backers of Mr Assad's government, while Turkey supports the armed opposition trying to topple him.
The two sides have traded blame for repeated violations of the December 30 ceasefire, which was also brokered by Russia and Turkey.
The Astana gathering is the first time Syrian government representatives have sat down with an opposition delegation made up mainly of rebel factions.
Previous face-to-face talks in Geneva included an opposition delegation made up mostly of political figures. During the last round of talks in Geneva in early 2016, Mr de Mistura was shuffling between the delegations sitting in separate rooms.